This invention relates to devices and methods for scanning beams of electromagnetic radiation, including beams of light and laser beams.
Precise and timely control of the direction of a laser or light beam is critical in many applications. In missile defense, these include the activities of searching for and identifying missiles, missile targeting and tracking, and rangefinding, aiming, and target imaging. In industrial applications, these applications include nano-fabrication, integrated circuit (IC) fabrication and repair, graphic displays, and laser material processing.
In-line, refractive or drop-in, beam scanners provide a compact, light weight, large clear aperture, and a rugged optical system compared with reflective scanners which require rerouting or xe2x80x9cdog-leggingxe2x80x9d of a laser beam to direct the beam into and out of a reflective scanner. Scanners having a large clear aperture that are also lighter and faster than existing scanners will improve the performance of laser radars, laser range finders, graphic display systems, imaging systems and any system employing laser beam scanners.
Most conventional beam steering/scanning devices use mirrors, which are well suited to making large (xcx9c180xc2x0) changes in the direction of polychromatic beams. However, it takes at least two mirrors to effect a minor change in the direction of the light beam, and at least three mirrors if the pivot point or center-of-scan of the beam is to remains on the original beam axis. At least one of the three mirrors must also be mounted off-axis. This adds to the size of the device, increases its weight and cost, and makes it inconvenient to use. If the laser beam is monochromatic, as most laser beams are, and if the desired deviation is no more than +/xe2x88x9230xc2x0 about a nominal beam direction, refractive beam steerers and scanners provide a number of advantages.
In many applications it is desirable to deviate the direction of a light beam rapidly and repetitively. Common prior art methods employ galvanometer drives to oscillate low moment of inertia mirrors about one axis, or to spin polygonal mirrors about a single axis. All mirror-based techniques produce a major deviation of the incident beam; yet, it is often desirable to produce a beam scanning motion without changing the basic direction of the beam. As described hereinafter, the present invention provides a device that can be inserted into the path of a light beam to scan the transmitted light beam over a range that remains centered on the original beam axis. The device can be retrofitted to existing optical systems without the need to reroute or redirect the beam.
Several types of refractive beam steerers already exist, including moving lenses, matched-lens adjustable (aka. lubricated adjustable optical) wedges, and rotating Risley Wedges. However, none of these prior art devices are capable of high scanning rates. One reason for this is that the moving components of these devices are not symmetric about an axis of rotation, and/or, they have relatively large moments of inertial which make rapid movements impractical because of the resultant high stresses, flexing and vibrations.
Referring to FIG. 1, an in-line refractive beam steerer/scanner, commonly referred to as a Risley Wedge Scanner or diasporometer is shown. The scanner uses two thin prisms or optical wedges W1, W2 which are in parallel with each other. In this device each prism is separately rotatable about the axis of an incoming beam B. Rotating wedge W1 sweeps an incident beam B around in a cone shaped pattern whose apex is on the original beam axis. In FIG. 1, the deviation of beam B to its original path, after having passed through wedge W1, is given as xcex41. Rotating wedge W2 allows the beam to be positioned anywhere within a cone angle that is approximately double that of a single wedge. The deviation of beam B, having passed through wedge W2, is given as xcex41+xcex42. If the two wedge angles are not exactly equal, then xcex41xe2x89xa0xcex42, and the wedges cannot return the beam to its original (undeviated) path. Further, even if the two wedges are exactly the same, the relationship between the resulting deviation and the position of the wedges is complicated by the fact that the deviation depends upon both wedge angle and angle of incidence. Rotating wedge W1 alters the angle of incidence on wedge W2 so the deviation produced by wedge W2 is generally not the same as that produced by wedge W1. This is why counter-rotated wedges do not produce a straight line scan pattern, but rather a xe2x80x9cbow-tiexe2x80x9d pattern instead.
Recently, a lubricated adjustable optical wedge (LAOW) has been introduced which overcomes blind spots, and the nonlinear and non-orthogonal behavior of Risley wedges. However, LAOW""s have heretofore not been able to achieve the high scanning speeds of Risley wedges. The present invention combines the optical function of a beam steering wedge in a device mechanically symmetric about an axis of rotation and therefore capable of high rotational speeds without producing excessively high stress, distortion or vibration.
Briefly stated, a beam steerer/scanner of the present invention utilizes two identically formed sections of transparent hemispheres (or hemi-cylinders or circular wedges) which are of the same size and shape, are made from two materials of equal density, but which materials have different refractive indices. The two pieces are joined together to form a dynamically balanced bi-index rotating element. When the element is oscillated or rotated, a transmitted beam impinging upon the element is deviated from its initial transmission path. Because the two materials are of equal density, the resulting assembly may be oscillated or rotated at very high speeds (6000 rpm) without causing excessive vibration or stress. The steerer/scanner provides a high ( greater than 0.95) throughput and scanning rates as high as 1 kHz are possible. A two-dimensional scanning unit can also be constructed using the same components as used to produce a one-dimensional scanner.
The steerer/scanner has an in-line, drop-in, design that saves space, weight and cost in a unit, while providing large clear apertures in constrained spaces such as missile domes. Finally, the steerer/scanner is especially effective in the infrared portion of the spectrum, where high refractive index materials are common.
Other objects and features will be in part apparent and in part pointed out hereinafter.